Its free to download and play. Right now there is a small number of song selection but according to their facebook page they will be constantly adding songs and categories.
The Game Play
You start by creating a game or continuing a recent match.
You can create games by facebook friends (that are also using the app), nickname (if you know their exact sing something nick/username), email (email used in registeringif not from facebook) or random (random users registered to the app).
After selecting your opponent/partner (actually i prefer the term partner since you need to cooperate to earn points). silly developers, anyhows after that, you will now choose the category of the song. If you have enough coins you can select other categories aside from the free "Best Songs of All Time".
You will then have to choose between 3 songs randomly given by sing something. If you dont know any of the song on the selection you can use the "bomb" to show another set of songs.
You are given i think 10 seconds to either Sing, hum or beat, or play an instrument to help your partner recognize the song. Singing gives x2 coins, x3 for no words just hum or beat, and x4 with instrument.
You gain coins if you or your partner has guessed the right song and artist combination
Hopefully they release a version for android phones and they add more songs and cheaper categories.
Here is their facebook page, their website, and a link to the itunes store.